The invention relates to a glass or glass-ceramic and to an advantageous use thereof. More particularly, the invention relates to a glass or glass ceramic that can be used as a conversion material for converting radiation of a first energy into radiation of as second energy, such as for converting blue light into white light.
Light sources are generally divided into discharge lamps and solid-state lamps. Among solid-state lamps, heat radiators dominate for general-purpose illumination and automotive applications, i.e. applications which require absolute brightnesses, e.g. halogen lamps. In addition, solid-state light sources in the form of luminescence radiators, such as for example inorganic LEDs, are known.
LEDs are generally highly advantageous, since they combine a number of properties including: a high efficiency as a result of direct conversion of electrical energy into light energy, compactness (punctiform radiators, which means that there is a wide range of design options for illumination systems), different colours (dynamic light matching and user-orientated illumination are possible using colour-mixing concepts).
Until a few years ago, however, LEDs were only used as low-emitting applications, in particular for displays. In recent times, however, the considerable potential of LEDs for applications with a higher demand for light has been recognized, and increased efforts have been made to achieve improved introduction of energy and an improved heat management in LEDs. However, more intensive utilization of LEDs for general illumination purposes or for use in automotive applications requires further adjustment to design and materials used with a view to:                further increasing the efficiency (into the range of florescent lamps, i.e. approx. 100 lm/W),        increasing the absolute introduction of energy in order to generate even greater brightnesses (50 to 2000 lm),        improving the discharge of light,        improving the conversion of high-power LEDs which emit in the blue or UV region, in order to produce as ideal a white colour sensation as possible,        improving the thermal and UV long-term stability of the materials used in an LED.        
LEDs generate light in a very narrow spectral region, whereas illumination purposes generally require white light. Commercially available white LEDs use a III-V semiconductor emitter to excite a luminescent material which emits a secondary wavelength in a lower wavelength region (down-conversion). One known option uses a blue InGaN/GaN LED to excite a broad-band, yellow phosphor, YAG: Ce. With these phosphor-converted LEDs, a certain proportion of the blue emission passes through the phosphor layer which covers the LED chip, so that the overall spectrum which results has a colour very close to that of white light. In this context, however, in most cases the colour is unsatisfactory owing to the absence of spectral components in the blue/green region and the red wavelength region.
A further approach consists in using a semiconductor emitter which emits in the UV or near-UV region and is coupled to a full-colour phosphor system. This allows white light sources of satisfactory colour to be realized (cf. Phys. Stat. Sol. (a) 192, No. 2, 237-245 (2002), M. R. Krames et al.: “High-Power III-Nitride Emitters for Solid-State Lighting”).
In this case, the phosphor particles are embedded in epoxy resin and applied as luminescent layer to the semiconductor emitter.
In the abovementioned phosphor layers, which are used to convert the light emitted by the LEDs in a desired spectral region, in particular to generate white light, certain drawbacks result from the fact that the phosphors used are embedded in epoxy resin. The granules used cause scattering losses. An inhomogeneous distribution of the granules on the semiconductor emitter can lead to different colour sensations depending on angle. Furthermore, epoxy resins in many respects lack long-term stability, in particular in terms of their optical and mechanical properties. The thermal stability is generally also insufficient to: generate high brightnesses. Moreover, the production of conversion layers of this type is complex and expensive.
Furthermore, it is known from JP 2001 214162 to use a phosphor which has an oxynitride glass matrix comprising 20 to 50 Mol % of CaO, 0 to 30 Mol % of Al2O3, 25 to 60 Mol % of SiO2, 5 to 50 Mol % of AlN and 0.1 to 20 Mol % of a rare earth oxide and a transition metal oxide, to generate white light by means of an LED which emits in the blue region.
DE 101 37 641 A1 discloses a hybrid LED which converts the light spectrum pre-dominantly in the UV region emitted by an LED into a longer-wave light spectrum by means of a luminescent glass body.
However, this publication does not give any details as to the structure of the luminescent glass body.
Although, in principle, luminescent glasses which are doped with rare earths and are used in particular in ophthalmology, as filters, in laser applications, for upwards conversion and for luminescence applications, are known, the prior art does not disclose any luminescent glasses which are suitable for generating white light with a sufficiently high quality and intensity to allow them to be used, of example, for indoor illumination purposes.
For example, JP 2000 281 382 A discloses silicate glasses and glass-ceramics which contain rare earth cations in order to generate the luminescence. These glasses and glass-ceramics contain from 30 to 70 Mol % of SiO2, up to 10 Mol % of GeO2, from 5 to 40 Mol % of MgO and from 10 to 55 Mol % of MO, where M is selected from Ca, Sr and Ba.
EP 0 847 964 A1 discloses an oxidic fluorescent glass which contains 2 to 60 Mol % of SiO2, 5 to 70 Mol % of B2O3 and 5 to 30 Mol % of RO, where R is selected from Mg, Ca, Sr and Ba. 2 to 15 Mol % of Tb2O3 or Eu2O3 are added for luminescence purposes.
U.S. Pat. No. 4,530,909 discloses an aluminosilicate glass which contains from 30 to 60 Mol % of SiO2, 20 to 35 Mol % of Al2O3 and 10 to 30 Mol % of an yttrium concentrate which predominantly comprises Y2O3 and also contains rare earth oxides as well as ZrO2.
This glass is extremely complicated to produce. Also, the glass does not have the required luminescence properties.